Various hybrid powertrain architectures are known for managing the input and output torques of various prime-movers in hybrid vehicles, most commonly internal combustion engines and electric machines. Series hybrid architectures are generally characterized by an internal combustion engine driving an electric generator which in turn provides electrical power to an electric drivetrain and to a battery pack. The internal combustion engine in a series hybrid is not directly mechanically coupled to the drivetrain. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine, and the electric drivetrain may recapture vehicle braking energy by also operating in a generator mode to recharge the battery pack. Parallel hybrid architectures are generally characterized by an internal combustion engine and an electric motor which both have a direct mechanical coupling to the drivetrain. The drivetrain conventionally includes a shifting transmission to provide the necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions thereby enabling high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and multi-mode operation.
Control of speeds within a multi-mode, compound split, electro-mechanical transmission is a basic operational requirement. When torque transmitting devices or clutches are engaged and disengaged, the number of degrees of freedom changes within the transmission. In an exemplary two-mode, compound-split, electromechanical transmission, during a neutral operation, there are three degrees of freedom where the vehicle speed defines one degree of freedom and the transmission control can control two other transmission speeds. During either of two modes of operation the transmission has two degrees of freedom where the vehicle speed defines one degree of freedom and the transmission control can control one other transmission speed. During a fixed-ratio operation, which is characterized by clutch applications effecting direct mechanical coupling of the input to the output, the transmission has one degree of freedom wherein the vehicle speed defines that degree of freedom and the transmission control cannot control any other transmission speed, and hence no speed control is necessary.